Compatibility of mixed zone constituents (YAG, YAP, YCrO₃) with a chromia-enriched TGO phase during the late stage of TBC lifetime

Kurzfassung

The effects of chromia accumulation in the TGO scale of late-stage MCrAlY-based EB-PVD TBC systems are addressed by comparing mixed zone microstructures from isothermal as well as cyclic oxidation tests at 1100 °C with phase relationships derived from the 1100 °C isothermal section of the ternary system Al₂O₃-Cr₂O₃–Y₂O₃. For this approach the c-ZrO₂ phase zirconia coexisting with mixed zone constituents is considered an excess component. Compacted powder pellets were processed via co-decomposition of nitric salt solutions. The reaction products were characterized by X-ray diffraction and Raman spectroscopy. Characteristics of the ternary Al₂O₃–Cr₂O₃–Y₂O₃ relevant for the mixed zone include (i) a continuous solid solution series between the orthorhombic phases YAlO₃ (YAP) and YCrO₃, (ii) a limited Cr vs. Al substitution (16 mol%) in Cr–YAG Y3(Al_1−xCr_x)₅O₁₂ and (iii) extended phase fields involving Cr-YAG and sesquioxide phases which represent typical late-stage mixed zones in the excess-zirconia model scenario. The YCrO₃ phase was found to be limited to very chromia-rich environments only. A high chromium enrichment of the mixed zone may be established upon extensive cycling through the miscibility gap in the alumina–chromia system. Later in TBC lifetime the thickness of the mixed zone is increasing due to outward growth of the Al_2−xCr_xO₃ phase into the bottom TBC layer which results in a morphological instability of the TGO/TBC interface.